Laser resonator

ABSTRACT

Laser resonator, particularly for carbon dioxide lasers, with two resonatornd mirrors having active material between two fully reflecting end mirror faces, which form an unstable resonator cavity, and with at least one third, completely reflecting mirror face reflecting the laser radiation prior to its coupling out through a coupling-out opening of the resonator. So that the laser only requires a reduced degree of coupling out and/or has an improved beam quality, it is constructed in such a way that, in addition to the unstable resonator cavity, between the two end mirrors is provided at least one stable resonator cavity and that the stable resonator cavity is made from the same active material of the third mirror face as the face of one of the two end mirrors with the face of the other end mirror.

DESCRIPTION

1. Technical Field

The invention relates to a laser resonator, particularly for carbondioxide lasers, with two cavity resonator end mirrors, which have activematerial between two fully reflecting end mirror faces, which form anunstable resonator cavity and with at least one third, completelyreflecting mirror face with which the laser radiation is reflected priorto its coupling out brought about by a coupling-out opening of theresonator.

The coupling out of laser radiation or illumination and therefore laserpower from the optical resonator of the laser is normally brought aboutin that a resonator mirror is either constructed in partly transmittingmanner, or by it radiating out from the resonator area laser radiation.Resonators with partly transmitting mirrors are preferably stableresonators, in which the laser beam as a result of the focussing actionof one or both mirrors remains localized around the resonator axis inthe area defined by the mirrors. Such a construction of stableresonators with rotational symmetry about the resonator axis has theadvantage that the coupled out laser radiation has a high beam quality.This is e.g. determined by the distance over which the laser beampropagates in an approximately parallel manner or is a measure for howsmall the focal spot can be on which the laser beam can be focussed orhow large is the maximum achievable intensity. The theoretically highestquality for the beam distribution is obtained according to the Gaussianpredominant mode TEM 00. Other beam distributions, e.g. TEM 01 havedecreasing beam quality with increasing beam diameter. However, a largebeam diameter is necessary to achieve high output power levels, if it isassumed that the latter require large active material volumes and theresonator length is limited for obvious reasons. Thus, in principle, anincrease in the output power by increasing the beam diameter is linkedwith a deterioration of the beam quality. In addition, the transmittingmirrors cannot be loaded to a random high level, i.e. must not beexcessively heated by the laser radiation passing through them, so thattheir optical effect is not impaired and their material is notdestroyed.

2. Prior Art

It is generally known to equip optical resonators with fully reflectingmirrors, which are constructed and arranged in such a way that a certainproportion of the laser radiation leaves the resonator area followingseveral revolutions. Coupling out conventionally takes place with apinhole or so-called scraper mirror, which produces the hollow beamcharacteristic of unstable resonators. Such unstable resonators normallyhave a high degree of coupling out and consequently presuppose activematerial with a correspondingly high amplification or gain. The beamquality is generally lower by a factor of 3 to 4. It is particularly lowif the degree of coupling out is kept low, e.g. by a correspondingdimensioning of the reflecting faces bringing about coupling out. Anadditional fundamental disadvantage of the unstable resonatorconstruction is the comparatively high adjustment sensitivity of itsmirrors and its sensitivity to optical reactions, i.e. with respect tolight reflection from the processing point into the resonator.

Apart from the generally known unstable resonator, U.S. Pat. No.3,681,709 discloses a laser resonator, particularly for carbon dioxidelasers, with two resonator end mirrors, which have active materialbetween two fully reflecting end mirror faces, which form a resonatorcavity, and with at least one third completely reflecting mirror facewith which the laser radiation or illumination is coupled out of theresonator. The third mirror face used for coupling the laser radiationfrom the resonator is constructed as the end mirror face of one of thetwo end mirrors. The resonator chamber between the two end mirror facesis stable and the third mirror face of one mirror is used exclusivelyfor coupling out from the stable resonator laser light oscillating inthe predominant mode. An unstable resonator cavity is not present in theknown resonator. The laser radiation is in fact coupled out by the thirdface of one end mirror directly on a focussing mirror of a focussingsystem.

DESCRIPTION OF THE INVENTION

The problem of the invention is to so improve an unstable resonator ofthe aforementioned type, that it makes do with a reduced degree ofcoupling out and/or has an improved beam quality.

This problem is solved in that between the two end mirrors there is atleast one stable resonator chamber in addition to the unstable resonatorchamber and that the stable resonator chamber in addition to theunstable resonator chamber is made from the same active material of thethird mirror face as the end mirror face of one of the two end mirrorswith the end mirror face of the other end mirror.

The mixed construction constituted by a stable and an unstable resonatoris important for the invention. The stable resonator cavity increasesthe average time spent by the radiation in the resonator, so that theradiation intensity therein rises. Consequently the active material isoperated more strongly towards saturation and the efficiency rises.There is consequently a decrease in the degree of coupling out, i.e. theratio of the coupled out power to the power produced in the resonator.Thus, unstable resonators provided with a stable resonator cavity can beused in laser systems with a comparatively low amplification or gain inorder to produce the same external laser beam power. In addition,through the influence of the stable resonator cavity, the internalstructure of the coupled out laser beam is so modified compared withconventional resonators, that a higher beam quality or a lower focalradius is obtained. This will be explained by embodiments hereinafter.

Advantageously the third mirror face as the end mirror face of one ofthe two end mirrors forms with the latter a unit or is a component ofsaid mirror. This leads to a fixed spatial association between the endmirror faces, which reduces the adjustment sensitivity of the resonatorand consequently increases the beam quality. The end mirror having thetwo mirror faces can in the requisite form be produced withcorresponding high precision by turning and milling machines, e.g. inrotation symmetrical manner. Acceptable effort and expenditure leads tothe necessary high surface quality and shape tolerance with lambda/10 tolambda/20.

Appropriately the third mirror face is at right angles to the resonatoraxis, which helps to bring about a symmetrical construction of the laserbeam and is also advantageous for the production of the third mirrorface or other mirror faces, e.g. if one of these forms a unit with thethird mirror face. Thus, there are rotationally symmetrical andconsequently advantageous mirror faces, it the third mirror face iscircular or annular.

A simple resonator arrangement from the constructional standpoint isobtained if the laser radiation can be coupled out as a hollow beamthrough a coupling-out opening with adapted cross-section and the thirdmirror face is located in the vicinity of said opening within the hollowbeam.

The invention also relates to a laser resonator, particularly for carbondioxide lasers, with a resonator formed by two fully reflectingresonator end mirrors and with a circular light passage opening at rightangles to the resonator axis in one of the end mirrors

Such a laser resonator is known from EU-OS 0 100 089. One end mirror ofthe resonator has a hole or pinhole, so that the laser radiation fromthe other end mirror can pass through said hole to the reflectingmirror, which reflects in parallel the radiation striking it by means ofa cone provided with spherical reflecting faces and a circularring-shaped, concave, spherical reflecting face connected radiallythereto onto the mirror provided with the hole. In this known unstableresonator, coupling out takes place with a scraper mirror positionedbetween the two end mirrors, so that the laser beam provided is astandard hollow beam.

In order to improve the aforementioned resonator in such a way that itonly requires a reduced degree of coupling out and/or has an improvedbeam quality, the light passage opening at right angles to the resonatoraxis is a coupling-out opening forming a solid beam. In thecross-section of said solid beam, the complete coupled-out laser poweris concentrated close to the resonator or longitudinal axis, so thatthere is a corresponding improvement to the beam quality, particularlywith far field observation of the intensity distribution over the beamcross-section. At the same time the laser resonator can have a simplerconstruction, because there is no need to deflect the light in theresonator for its basic construction.

An advantageous construction of the resonator is obtained in that theend mirror having the circular opening has a circular ring-shaped convexor concave end mirror face facing the other end mirror and that theother end mirror has a circular ring-shaped concave or convex end mirrorface provided with an internal diameter tending towards zero. Thefundamental condition for the shaping of the two end mirror faces ismerely that, for producing the solid beam, they produce no reflectionbeams migrating radially outwards from the reflection area starting fromthe resonator axis, so that the entire coupled-out laser radiation isavailable with the solid beam. However, it is also possible to formstable resonator cavities cooperating with the resonator area formed bythe two end mirror faces and which are superimposed. This is achieved inthat radially outwards and/or inwards on the circular ring-shaped endmirror face of the end mirror having the opening is connected an endmirror face forming a stable outer and/or inner resonator chamber withthe concave face of the other end mirror. In the case of such resonatorsuse is made of the two aforementioned basic solution ideas together inthe sense of reducing the degree of coupling out and/or improving thebeam quality.

In the case of resonators with fading out of the solid beam, it isimportant to eliminate local fluctuations of the laser radiationintensity, which result from the fact that on passing through theresonator the beam is directly phased out from the outside to the centreaxis of the resonator. Such fluctuations are counteracted in that atleast one first end mirror has a face enabling a radiation fractionreflected by the second end mirror to be reflected over the centre axisof the resonator onto the second end mirror. The aforementioned featureslead to a coupling between areas of an end mirror or both end mirrorsfacing diagonally over and beyond the centre axis, if the mirror face iscorrespondingly constructed.

A simple construction of the resonator arrangement with respect to theend mirror face used for coupling is obtained if said face has a concaveconstruction and is arranged concentrically to the centre axis and formswith a circular ring-shaped end mirror face arranged directly round thelight passage opening a stable resonator cavity. Apart from reducing thefluctuations of the laser intensity emanating from the unstableresonator area in the case of coupling out of the solid beam, thisstable resonator cavity leads to an improvement of the beam quality.

A constructionally simple embodiment of a stable resonator is obtainedin that the second end mirror having the circular light passage openinghas a planar scraper mirror and the other end mirror a circularring-shaped, concave end mirror face. The planar scraper mirror issimple to produce for all the necessary dimensions, which isparticularly advantageous if, following the production of thecomparatively complicated first end mirror, it has to be matched to thelatter.

In order to avoid quality problems to the laser radiation by points,tips, etc. of the first end mirror located in the vicinity of the axisof symmetry, facing the light passage opening of the second end mirrorthe first end mirror is provided with a plane surface passingtangentially into the curvature areas of the concave end mirror faces.

As a result of the cooperation of a plane mirror and a convex mirror,the previously described embodiment constitutes a stable resonator.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described hereinafter relative to embodiments and theattached drawings, wherein show:

FIG. 1 the beam guidance within a first resonator according to theinvention.

FIGS. 2a to 2c possible variants of the mirror M2 in FIG. 1.

FIGS. 3a to 3c the near field intensity distributions I=f(r) and the farfield intensity distribution associated therewith in diagrammatic form.

FIG. 4 a radially integrated intensity distribution I=f (radius).

FIGS. 5 to 7 further embodiments according to the invention.

FIG. 8 another inventive embodiment avoiding laser intensityfluctuations.

FIG. 9 inventive embodiments with a plane scraper mirror.

BEST WAYS TO PERFORM THE INVENTION.

The resonator 10 shown in FIG. 1 comprises two rotationallysymmetrically constructed mirrors M1 and M2. In the case of acorrespondingly rotationally symmetrically constructed active material40, shown in dot-dash manner, said mirrors can be used with particularadvantage. However, a different construction of the mirrors M1,M2 isalso possible, e.g. rectangular, so as to be able to use in optimummanner active material 40 with a corresponding rectangularcross-section.

The dimensions of the mirrors M1,M2 are designated D, d1 and d2 and givethe diameters for said rotationally symmetrical mirrors, which togetherform the resonator axis 11. The mirror M1 is a concave mirror with acorresponding fully reflecting, concave mirror face 12, which is e.g.metallic. The mirror M2 is also metallic and therefore fully reflectingand has a convex mirror face 13. The two fully reflecting mirror faces12,13 form an unstable resonator or resonant cavity 10', i.e. the lightbeams reflecting between them do not remain in the cavity 10' or axiallyparallel and instead migrate radially outwards, as indicated by arrows14, so that there is a coupling out of the resonator 10 by an annularcoupling-out opening 15 in FIG. 1, so that the coupled-out laser beam 16is in the form of a hollow beam or with a so-called ring mode. The laserbeam 16 is shaped optically, e.g. collimated outside the resonator 10and supplied to a particular use, e.g. for welding.

However, apart from the mirror face 13 with the diameter d2, the mirrorM2 has a third mirror face 17 with an external diameter d1. The thirdmirror face 17 is planar and at right angles to the resonator axis 11,so that it is directed onto the face 12 of the mirror M1. It formstogether therewith a stable resonator cavity 18, which is hollowcylindrical and stressed by dotting in FIG. 1. In said stable resonatorcavity the laser radiation is produced with a mode having acomparatively high beam quality or a smaller focal radius of its laserradiation than in the case of such radiation between the mirror faces12,13, which will be explained relative to FIGS. 3a to 3c and 4.

In FIG. 3a I=f(r) represents the so-called Gaussian predominant mode TEM00, i.e. the intensity distribution is in accordance with a Gaussianerror curve. This representation applies to the intensity distributionin the near field of the laser or resonator. Also in the far field thelaser intensity is concentrated according to FIG. 3a on the paraxialradius regions. The representation relates to the intensity distributionin the focus of a 20 cm lens. FIG. 3b shows comparable representationsfor a hollow beam I=f(r) in the near field and diagrammatically in thefar field. It is clear that in the far field, which is determinative forthe relationships of the laser beam at the point of use, although thereis a concentration of the intensity in the vicinity of the beam axis, itis clear that the intensity is still considerable at greater spacings.The inventive resonator construction here leads to an improved resultwith reduced intensities outside the central intensity cone. This can begathered from FIGS. 3c and 4, where the value free-related integratedpower is shown as a function of the laser beam radius in the focus of alens (focal length 20 cm). The beam radius is fixed in the conventionalway defining that 86 % of the laser power is within a circle with saidradius. This gives the shown paths for the ideal case of TEM 00 inbroken line representation, for the conventional unstable resonator withM=2 corresponding to the continuous line curve and for an inventiveresonator with a stable cavity corresponding to the dot-dash line curve.Line 19 shows that the radius is reduced by up to 40% for the intensitydistribution in the case of the inventive resonator.

The resonators 10 shown in FIGS. 5 to 7 in each case comprise the twofully reflecting mirrors M1 and M2, which face one another and form theresonator axis 11. The mirror M1 has a circular ring-shaped, concavemirror face 12' with the associated radius r12' and an associatedcurvature centre k12'. In the vicinity of the resonator axis 11 themirror face 12' is rounded, in order to avoid a point and therefore aninhomogeneity location for the reflection of light.

The other mirror M2 is circular ring-shaped and has a light passageopening 20 at right angles to the resonator axis 11. The face facing themirror M1 comprises a convex mirror face 13' and a third, planar mirrorface 17. Therefore the latter forms with the mirror face 12' a stableresonator cavity 18, which is hollow cylindrical. In the interior of thehollow cylindrical, stable cavity 18, the radiation path issubstantially determined by the convex mirror face 13', whose radiusr13' can be calculated from the associated curvature centre k13'. Fromthe comparison of the position of the curvature centres k12',k13', it isclear that the resonator is unstable with respect to the intermediatelyreflected radiation. Consequently it is clear from the beam path shownin the lower half of FIG. 5, that within the unstable resonator cavity32 between the stable resonator cavity 18 and the beam path 31,reflections occur between the mirror faces 12',13', whilst the beamfraction characterized by the paths 31,33 is used for forming the fullor solid beam 34, which is therefore coupled out by the light passageopening 20 acting as a coupling-out opening. Due to the rotationallysymmetrical construction of the mirrors M1,M2, which is indicated by thearrow 35, the intensity distribution in the solid beam 34 is alsorotationally symmetrical.

The resonator 10 of FIG. 6 differs from that of FIG. 5 in that betweenthe mirror face 13' and the opening 20 there is a further mirror face17' acting in the same way as the third mirror face 17. This leads to afurther stable resonator cavity 18', which increases the beam qualityand reduces the degree of coupling out relative to the full beam 34.

In the resonator 10 of FIG. 7 two mirrors M1,M2 are shown, which have nomirror faces for forming a stable resonator cavity. In spite of thisthere is an improvement to the beam quality and a reduction to thedegree of coupling out due to the fact that the laser radiation iscoupled out as a solid beam 34. Thus, there are only mirror faces12',13', which satisfy the conditions for an unstable resonator.Otherwise the mirror faces 12',13' or the mirrors M1,M2 are constructedin the manner described relative to FIGS. 5 and 6. A full beam 34produced with the resonator according to FIG. 7 will not have thequality of the full beams 34 of the resonators of FIGS. 5 and 6, but itis clearly improved compared with conventional hollow beams. In thecases of FIGS. 1, 5 and 6, it is possible to construct the mirror faces17,17' determining the stable resonator cavity 18,18' differently fromthose constructed, provided that the conditions for a stable cavity arefulfilled. Examples for this are shown in FIGS. 2a to 2c. Whilst FIG. 2ashows a side view of the mirror M2 of FIG. 1, i.e. with a centrallyarranged convex mirror face 13 and a third, planar mirror face 17arranged in annular manner around the same, the third mirror face 17according to FIG. 2b is arranged in the centre of the mirror M2. Themirror face 13 is constructed as a concave ring, which is connectedradially outwards to the mirror face 17 and, apart from forming anunstable resonator cavity 10', is used for coupling out the laserradiation or illumination. This avoids a superimposing of the beam pathsand a resulting beam quality reduction. According to FIG. 2c the mirrorface 13 leading to the instability of the resonator 10 is positionedcentrally, i.e. as in FIGS. 1 and 2a. However, this is followed by athird, concave mirror face 17, which focuses the light, so thatmodifications of the mirror face 12 relating thereto can be carried out.Such mirror faces or mirror M2 can be constructed in the most variedcombinations with the other mirrors M1, provided that it is ensured thatthe beam quality is improved and/or the degree of coupling out is inparticular reduced by corresponding stable resonator cavities.

The resonator 10 shown in FIG. 8 fundamentally has a construction likethat of FIG. 7. Thus, reference should be made to FIG. 8 in connectionwith the description of mirrors M1,M2 and their faces 12',13'. Inaddition, there is a diaphragm 38 outwardly bounding the resonatorcavity. The special feature of the resonator 10 in FIG. 8 is that themirror M1 has a concave mirror face 37, which is located in the vicinityof the central tip of the mirror M1 in FIG. 7 and cooperates with amirror face 17" of the second mirror M2. This mirror face 17" isconnected directly to the light passage opening 20 for the coupled-outsolid beam 34 and is circular ring-shaped. It forms a stable resonatorcavity 18" with the mirror face 37. With the aid of the cavity 18", aradiation fraction reflected by the second mirror M2 is reflected beyondthe centre axis 11 of the resonator 10 and with the aid of the mirrorface 37 onto the second mirror M2.

FIG. 9 diagrammatically shows the essential components of a stableresonator 10, namely the two end mirrors M1,M2. The mirror M2 is aplanar pinhole mirror, whose hole forms a light passage opening 20 forthe solid beam 34. The end mirror M2 which symmetrically faces it withrespect to the resonator or symmetry axis 11 is provided with a circularring-shaped, concave end mirror face 12'. Between the latter and the endmirror face 13' takes place the laser process occurring in the not shownactive material, the diaphragm 38 forming an outer radial boundary. Theoptical axis 43 of the mirror system is ring-like with a diameter d andthe diameter d3 for the opening 20 for coupling out a beam fraction isadapted thereto and is namely slightly smaller.

The resonator of FIG. 9 is a stable resonator, which is due to the factthat the mirror M2 has an infinitely large radius of curvature, so thatthe curvature centre of the mirror face 12' is located between theinfinitely remote curvature centre of the mirror M2 and the same andconsequently fulfills the condition for more stable resonance.

The central mirror tip 39 shown in FIG. 9 can disturb the oscillatingprocess and can therefore lead to a deterioration of the beam quality.Therefore the end mirror M1 is provided with a plane surface 41, whichavoids such interference. The plane surface 41 issues tangentially intothe curvature areas 42 of the mirror M1, so that here again there are nodisturbing inhomogeneities of the mirror face 12.

INDUSTRIAL USABILITY

The inventive laser resonator is used for so improving an unstableresonator, that it only requires a reduced degree of coupling out and/orhas an improved beam quality.

We claim:
 1. A laser resonator for a carbon dioxide laser, comprisingtwo resonator mirrors placed on ends of an excitable active medium, atleast one of the resonator mirrors having at least two fully reflectingsurface zones with different curvature radii, wherein, between the tworesonator mirrors, at least one stable resonator chamber is provided bysurface zones of both resonator mirrors in addition to an unstableresonator chamber formed with other surface zones of the resonatormirrors, and radiation between the resonator mirrors is coupled outthrough an opening in one of the resonator mirrors.
 2. The laserresonator according to claim 1, wherein the surface zone of the at leastone mirror is at a right angle to a resonator axis.
 3. The laserresonator according to claim 1, wherein the surface zone of the at leastone mirror is annular.
 4. The laser resonator according to claim 1,wherein one of the mirrors is a feedback mirror smaller than the othermirror, laser radiation being coupled out around the other mirror andhaving an annular intensity distribution, and the surface zone of the atleast one mirror is at the periphery of the feedback mirror.
 5. Thelaser resonator according to claim 1, wherein the mirrors are fullyreflecting mirrors, one of which has a light output opening, and laserradiation out coupled from the output opening forms a solid beam. 6.Laser resonator according to claim 5, characterized in that the secondend mirror (M2) having the circular light passage opening (20) is aplanar pinhole mirror and the other end mirror (M1) has a circularright-shaped, concave end mirror face (12').
 7. Laser resonatoraccording to claim 6, characterized in that facing the light passageopening (20) of the second end mirror (M2) facing the first end mirror(M1) is provided with a plan surface (41), which passes tangentiallyinto the curvature areas (42) of the concave end mirror face (12'). 8.The laser resonator according to claim 5, wherein one of the mirrorshaving the opening is an end mirror having an annular-shaped fullyreflecting surface zone facing the other mirror and provided by rotatingone of a convex and a concave arc around a resonator axis, and the othermirror is an end mirror having a circular ring-shaped fully reflectingsurface provided by rotating one of a concave and convex arc and arealso having an inner diameter which approaches zero.
 9. The laserresonator according to claim 5, wherein, at least one of radiallyoutwardly and inwardly to an annular shaped fully reflection zone of thefirst-mentioned end mirror having the opening, a fully reflecting zoneforming a stable resonator chamber is tangentially connected with acorresponding fully reflecting zone of the other end mirror.
 10. Thelaser resonator according to claim 5, wherein, the fully reflecting zoneof the second-mentioned end mirror is formed such that, in crosssection, a laser radiation fraction outgoing from an upper part of thefirst-mentioned end mirror is reflected and incidences in a zone overand beyond a center axis of the resonator onto a lower part of thefirst-mentioned mirror and vice-versa.
 11. Laser resonator according toclaim 6, characterized in that the end mirror face (37) is concave andis arranged concentrically around the centre axis (11) and with acircular ring-shaped mirror face (17") arranged directly around thelight passage opening (20) forms a stable resonator cavity (18").